The present invention pertains to wastewater treatment, and, in particular, to an apparatus that introduces oxygen into the wastewater to increase the flocculation rate of suspended solids.
Sewage treatment by means of flocculation removes dissolved and suspended solids from wastewater. Such treatment frequently takes place at a wastewater treatment facility in a rectangular secondary tank downstream from the primary treated wastewater, which tank may be continuously seeded with bacteria-laden activated sludge to augment growth of the bacteria that oxidizes the organics and inorganics in the wastewater to achieve flocculation of the waste. Flocculate of activated sludge depends upon gravitational effects and settles on the tank bottom from where it is scraped off and pumped to another tank or digester for a subsequent anaerobic treatment or disposal. The flocculation rate depends upon the biological aspect of the solids being treated, and the environment and life supporting habitat within the wastewater for the bacteria that in effect devours the organic carbon and other suspended solids.
In particular, in order to increase the rate of flocculation of the activated sludge, wastewater treatment processes frequently involve aeration to provide oxygen for the growth of the bacteria seedings. The provided oxygen is diffused into the wastewater and in combination with the dissolved oxygen provides life support for the bacteria that produces the flocculation of the waste. The replenishment of suspended oxygen along with the dissolved oxygen fosters the respiration of the bacteria that impacts the retention time needed for flocculation, which time is based upon numerous other factors including the characteristics of the sewage being treated, hydrostatic head, compressed air discharge temperatures, and net diffuser discharge pressure.
One known wastewater aeration system is diagrammatically shown in FIG. 1. Concrete reservoirs or baths 10, each of which may be one of many in a series of similarly constructed baths, each holds a quantity of wastewater 12, such as domestic sewage, being treated. Air supply pipes 18 include at their respective downstream ends a diffuser section 16 submerged within the wastewater 12. The diffuser sections 16 include a multitude of small orifices through which air is output or bubbled into the wastewater. Although only one supply pipe and diffuser for each reservoir is shown in FIG. 1 for illustration purposes, multiple pipes and/or diffusers may be used to provide an adequate amount and distribution of aerating air. Each of the air supply pipes 18 is connected to a distribution manifold 20 connected to a large diameter duct 22 that is supplied with pressurized air from a blower or centrifugal compressor 24. A filtered intake duct 26 that ports or opens to the outside where ambient air is connected to compressor 24.
During operation, compressor 24 draws ambient air into duct 26, which air then passes in sequence through compressor 24, duct 22, manifold 20, and supply pipes 18. The blower conveyed air continues out through diffuser sections 16 in the form of bubbles, shown at 30, that bubble or percolate upward through wastewater 12 to provide oxygen for the respiration of the bacteria within the wastewater.
While the prior art aeration system shown in FIG. 1 does on occasion provide some benefit, its effectiveness at treating the wastewater is sometimes limited. Specifically, the respiration of the bacteria in the wastewater is at optimum in a habitat of around 68xc2x0 F. (20xc2x0 C.) to a high of around 140xc2x0 F. (60xc2x0 C.).
Except for some rare heat resistant strains of bacteria, higher temperatures may harm or kill the bacteria within a short period of time. However, in the prior art of FIG. 1, the heat of compression incidentally applied to the aerating air by the blower may, depending on ambient air conditions, increase the temperature of the air reaching the diffuser and the wastewater to undesirably high levels at which bacteria respiration is hindered, or the bacteria is killed, and the solubility of dissolved oxygen is diminished. For example, in some situations and blower configurations, air output from the diffuser will have a dry bulb temperature of in excess of 155xc2x0 F. when ambient air has a dry bulb temperature of as low as 70xc2x0 F. Although such diffuser output temperatures may not measurably change the overall temperature in the tank so as to adversely affect all the bacteria therein, bacteria passing near the diffusers will be subjected to thermal shock.
Another shortcoming of existing aeration systems is related to the net diffuser discharge pressure, which is the difference between the discharge pressure at the diffuser orifices and the wastewater hydrostatic pressure. The pressure delivered by a centrifugal blower is dependent on the intake ambient air mixture density and specific humidity, which density is a function of the temperature and humidity of the ambient air. Under some ambient conditions, such as high temperature with high humidity, pressure losses due to high temperatures and vapor condensation downstream of the blower result in the net diffuser discharge pressure being insufficient to produce bubbles or to prevent wastewater from clogging the air distribution pipes. Under certain other ambient conditions, the net diffuser discharge pressure is so high that the diameter of the diffused air bubbles are too large and the wastewater is made too turbulent for efficient absorption of oxygen into the wastewater.
Still another shortcoming of existing aeration systems is related to the fact that classical adiabatic blowers ingest air at constant volumes. Because the density of air varies with temperature, the mass of air conveyed by the blower varies with temperature. As the temperature and relative humidity of air inlet to the blower increases, the mass of air and therefore oxygen delivered to the diffuser for introduction into the wastewater for use by the bacteria undesirably decreases. And, although the horsepower required to compress this less dense mass flow is reduced, its rate of decline is much less than that of the mass flow.
Thus, it would be desirable to provide a wastewater aeration system that overcomes these and other shortcomings of the prior art.
The present invention provides a wastewater aeration system that improves the flocculation rate of suspended solids within the wastewater by conditioning an intake air flow prior to its introduction into the compressor of the aeration system. The intake air is conditioned by a vapor condensing heat exchanger within the air stream that de-hydrates and cools the air to a desired level. Another and optional heat exchanger positioned within the air stream more downstream of the vapor condensing heat exchanger will secondarily sensibly cool the air stream upstream from the bell-mouth of the compressor. The inventive heat exchangers may also be configured to heat the air stream when ambient conditions are cooler than desired.
In one form thereof, the present invention provides a wastewater treatment system for a bath of wastewater, including a conduit having an outlet opening into the bath of wastewater, an air duct having an inlet in flow communication with a source of air, at least one blower in flow communication with the air duct and the conduit and operable to cause air to pass through the air duct and through the conduit to be output through the conduit outlet into the bath of wastewater, and a heat exchanger installed in the air duct and adapted to condition air passing through the air duct.
In another form thereof, the present invention provides a wastewater aerating system including a conduit having at least one outlet submerged within a bath of wastewater to be treated, an air duct having an inlet in flow communication with a source of ambient air, at least one blower in flow communication with an air duct outlet and a conduit inlet and operable to cause air to pass through the air duct and through the conduit to be output through the at least one outlet of the conduit into the bath of wastewater, and at least one heat exchanger installed in the air duct and adapted to condition air passing through the air duct, whereby the temperature of the conditioned air reaching the at least one blower is modulated by the at least one heat exchanger to promote flocculation of suspended solids from the wastewater when the conditioned air is output into the bath of wastewater.
In still another form thereof, the present invention provides a method of treating a bath of wastewater including the steps of providing a conduit having an upstream end in communication with a source of ambient air and a downstream end opening into the bath of wastewater below the wastewater surface, inletting ambient air into the conduit through the upstream end, moving the inlet air through the conduit from the upstream end to the downstream end, outletting air from the conduit into the bath of wastewater through the downstream end, and conditioning the air moved through the conduit such that the air outlet from the conduit is at a temperature that promotes flocculation of suspended solids from the wastewater.
One advantage of the present invention is that it provides a water aeration system that is cost effective to operate.
Another advantage of the water aeration system of the present invention is that it either reduces compressor brake horsepower required to aerate a given tank, or results in an increased compressed air output at the diffuser in the tank for a given installed compressor horsepower,
Another advantage of the water aeration system of the present invention is that by increasing the amount of suspended oxygen that is introduced into the wastewater and thereby improving the habitat for the flora of bacteria therein, the flocculation rate of suspended solids is increased so as to essentially increase the capacity of the wastewater plant by decreasing the required retention time of the wastewater within the plant.
Another advantage of the present invention is that it provides a water aeration system that within a normal range of ambient air conditions will deliver air at a relatively constant net diffuser discharge pressure and temperature to a diffuser immersed in a bath of wastewater, thereby providing a suitable air quality for suspending oxygen into the wastewater, a reduction of thermal shock on the piping system, an even flow of air through the diffuser, a reduction of head loss due to air-side fouling, a reduction of power consumption due to liquid-side fouling, a suitable oxygen transfer efficiency to the wastewater, a suitable bubble formation with reduced vapor content at the diffuser, and a reduction in vapor condensation in the piping.
Another advantage of the water aeration system of the present invention is that it delivers air to the diffuser at temperatures less likely to create a thermal shock to the flora of bacteria.
Another advantage of the present invention is that it provides for an increased suspended oxygen supply to the flora of bacteria their respiration temperature.
Another advantage of the present invention is that it may lower the temperature of the air stream outlet at the diffuser, thereby lessening the turbulence of the water column and consequently increasing the settle rate of flocculate from the wastewater.
Still another advantage of the present invention is that it controls the diameter of the diffused air bubbles, thereby permitting a less violent action of bubble formation for a more efficient absorption rate of oxygen into the wastewater.
Still another advantage of the present invention is that it reduces turbulence and churning of the wastewater which diminishes the stillness required for sedimentation of the flocculate.